THE story of our 2-million-year journey towards becoming human has been told so often it’s almost a cliché. But despite a vast stack of books and papers, there are still questions to which the answers are sketchy or controversial.

It has, for example, never been made clear why it all began in east Africa, not elsewhere. After all, Europe and Asia both had apes too, so why did they not start the transition to something human, allowing us to descend not from Australopithecus, but from Gigantopithecus, the 3-metre-tall distant relative of the orangutan?

Enter Mark Maslin, professor of climatology at University College London and his new book, The Cradle of Humanity. In it he addresses many outstanding questions, while showing what was going on in the world at the time. As a climatologist, he looks at the ecological consequences of the great dry-out that occurred not only in east Africa but also in primate-rich Europe and primate-free North America.

In most accounts these events receive the broadest of historical brushstrokes, with just enough detail to lend a plausible inevitability to whatever process is under discussion. The Cradle of Humanity is more textured and subtle, showing not only how such changes altered any meat-giving prey, but how both climate and new mammalian fruit eaters changed the suite of plants available to our early ancestors. Only then does Maslin tackle the probable consequences of this for the social systems and mental development of proto-hominins.

But this book offers far more than a palaeoanthropological cocktail with a twist. Maslin dedicates whole chapters to the history of Earth and its climate, as well as showing how the interaction of wobbles in the orbits of our planet and the moon create climatic cycles. Then there are the effects of plate tectonics, rain shadows and lakes of varying ephemerality and salinity.

All this allows Maslin to buttress his central contention, that human evolution as we know it wouldn’t have occurred without the uplift of the Tibetan plateau and the formation of the Great Rift valley. These events, and the cycling between salt flats and shallow sea that mark the history of the Mediterranean, are the great drivers of human evolution – the climatic starting gun that set off the human race.

Maslin also provides a fine overview of the evolution of evolutionary thinking over the past 150 years, to the point where we now see it less as an orderly march towards an inevitable Homo sapiens and more of a random stumble to now. He is clear that while the appearance of a smart, tool-using primate is no major surprise, the presence of this particular smart, tool-using primate, arising as a result of that exact evolutionary trajectory, owes much more to chance and contingency than previous popular perspectives allowed.

“Europe and Asia both had apes too, so why did they not start the transition to something human?”

For much of early human history, for example, there was another smart bipedal ape on the African savannah: Paranthropus, a heavy-jawed grinder of nuts, seeds and tubers. Even a tiny disaster could have wiped out the protohuman’s prey base, leaving only Paranthropus. Similarly, the cycles of aridity and plenty could have been very different, given a greater or lesser slippage of ice fields into the ocean, say, or a change in when the Strait of Gibraltar closed and the Mediterranean experienced death by evaporation. Such shifts might have had us reading this on Mars, or squatting round a cave fire.

In synthesising the most recent research in palaeoanthropology and giving the ecology of our ancestors a climatological twist, Maslin has produced a book that is fascinating, humbling and informative.

This article appeared in print under the headline “How the world made us”